Symptoms of grape fanleaf disease (shown on the leaf on the left) include vein banding and yellowing, possibly resembling herbicide-damaged leaves.

Washington State University

Grapevine viruses are the silent killer of vineyards. They lurk in unclean propagative material, the mouths of tiny vectors, and old root material, and wait to take hold in a vineyard, reducing yield and quality. And worse, state programs designed to produce clean ­material are not ­fail-safe.

The grapevine, a “treasure trove” for viruses, can be infected by more than 70 viruses worldwide, said Washington State University’s Dr. Naidu Rayapati. Fortunately for Washington State grape growers, not all 70 are found in the state, with only a few causing major problems: grapevine leafroll disease, rugose wood disease complex, and grapevine fanleaf virus.

Until recently, grapevine leafroll disease was thought to be the primary disease that growers in Washington had to worry about. Since 2005, WSU has analyzed some 2,000 samples for different viruses from random vineyards throughout the state, and while the majority of detections are from the grapevine leafroll complex, three of five rugose wood complex viruses have been found, along with fanleaf and a few minor viruses.

Symptoms

“Washington’s virus symptoms don’t always match the classical, textbook symptoms,” Rayapati said during a recent grape disease seminar sponsored by the Washington Association of Wine Grape Growers and WSU. For example, the rugose wood complex are trunk diseases, and symptoms occur when scion wood is grafted on an indicator host.

Rayapati explained that because Washington growers use own-rooted vines, many of the virus symptoms are not as clearly expressed as on grafted vines. Additionally, virus symptoms are highly variable, and can look different from one variety to the next. Generally, the following symptoms should make a grower suspect disease:

Leafroll—Red leaves with green veinal area in red varieties, downward curling of leaves in white varieties. Red veins in Cabernet Sauvignon instead of green veins have also been seen.

Fanleaf—Vein banding, yellowing of leaves or yellow mosaic-type symptoms. Leaves are smaller than normal; clusters are also smaller. May look like herbicide damage or genetic abnormalities of the leaves.

Rugose wood complex—No symptoms on own-rooted vines. On grafted vines, the woody cylinder of the scion or rootstock near the graft union is marked by pits and/or grooves.

Disease symptoms can also mimic nutritional deficiencies, mechanical trunk or vine injury, or cold injury, he added. “You need to consider other factors before you jump to the conclusion that red leaves mean virus.”

Testing

The only surefire way to know if viruses are present is through accurate diagnosis. The WSU virology laboratory uses both PCR (polymerase chain reaction) and ELISA or enzyme-linked immunosorbent assay tests to look for virus. Rayapati encouraged growers who believe they may have virus in their vineyard to contact him and he can take samples as part of his statewide testing. Results are confidential.

With fanleaf, symptoms are easily seen in the spring, though not fall. For leafroll disease, symptoms are best observed after veraison. “But you can detect leafroll virus throughout the season from bud break to harvest, irrespective of whether you see symptoms or not,” he said, adding that ELISA testing may or may not pick up fanleaf if fall samples are collected. For rugose wood complex, PCR is the only testing technique that can be used.

Grapevines often have more than one disease, which means the lab must look for multiple diseases during ­testing.

Short- and long-term problems

In the short term, leafroll-infected vines produce less fruit and the fruit is lower quality than fruit from healthy vines. Rayapati tracked infected Merlot grapevines for several years and found annual yield reductions ranging from 15 to 30 percent compared to healthy vines in the same vineyard. Infected vines had fewer clusters and berries than healthy vines.

Wine quality was impacted, too. He found that the Brix level was 10 percent lower and acidity was higher in infected Merlot grapes. Wines made from the infected grapes had significantly lower anthocyanins, tannins, phenolics, and alcohol content than normal grapes. “Overall, the wines were more acidic and less fruity.”

A bigger problem in the long term is the blanketing of the disease throughout the vineyard. What starts as a few infected vines can eventually spread throughout the block. Grape mealybug and some scale insects are known to spread leafroll disease. Just being located across the road from an infected vineyard can be enough for a clean vineyard to become infected.

Be clean, stay clean

Be clean and stay clean is Rayapati’s mantra, but it’s not easily accomplished. The first line of defense, he said, is using virus-free plant material.

Buying certified material is supposed to mean it is certified free of virus. But which virus(es) and when was the mother block or registered block last tested? Certification programs vary state by state, with different viruses targeted for elimination. Viruses can move into clean mother or registered blocks. Close monitoring and routine testing is needed to stay on top of changes.

He advises growers to test plant materials before planting if there is any concern about infection, he said. That includes cuttings from a vineyard for propagation.

After planting, monitor the new vines closely for any signs of problems. “If you see a problem, take quick action to identify if it is in fact due to virus,” he said, adding that infected vines should be removed, along with one or two vines on both sides of the infected vine. Remove as much root material as possible.